The UV curing of resin formulations is widely used in industry as the setting process for coatings, adhesives, and more recently paints. Such formulations may comprise a combination of vinyl, usually acrylate, monomers and crosslinkers, together with a photoinitiator. Other possible constituents of the formulations include crosslinkers and vehicles. In general an advantage of photocurable formulations is that the monomers act as their own vehicle, and the use of solvent is obviated, which has environmental advantages.
Advances in the technology of photocuring, improvements such as, those in UV lamps, cationic initiators for epoxide-based formulations, water borne coatings, and many novel monomers has enabled this production process to penetrate a number of important manufacturing sectors. Photopolymerization is now used in photo resists for printed circuits and microelectronics, for photolithography, magnetic recording media, glass-fiber laminates, and for medical devices, especially for dental and ophthalmic applications.
European Patent 0800 657 describes a photoinitiator linked to a macromer structure which together with a copolymerizable monomer and a crosslinker is capable forming a polymerization product, such as an ophthalmic lens that retains photoinitiator radical in the resulting network. This is advantageous in medical applications wherein such potentially harmful radicals must be carefully controlled. However, this system would not be applicable for producing a polymerized product directly in the capsular bag in the eve since it is not directed to photoinitiators activated by light in the visible range.
For the medical applications of photopolymerization it is usual to employ visible light, rather than UV, to effect the cure of the resin formulation. The use of visible, usually blue, light avoids exposing patient and dentist or surgeon to harmful irradiation. Increasingly, the merit of this approach is being recognized for industrial practice, where operatives also need protection from prolonged exposure to harmful UV.
It is a characteristic of almost all, if not all, of the formulations used for aforementioned types of application that they are crosslinked. Crosslinking of the polymeric bases, which constitute the coatings, or artifacts of the aforementioned industrial products confers important advantages upon them. Crosslinked polymers have greater environmental (e.g. temperature and moisture) resistance, solvent resistance and dimensional and mechanical stability, than equivalent linear polymers. This is especially so for where the equivalent linear polymer are produced by photopolymerization they have an atactic, non-crystalline, structure.
Crosslinking is introduced into photopolymerized products by including in the formulation for the resin, coating or gelling system, an acrylate, or similar, crosslinker, which is characterized by having two or more crosslinkable acrylate or vinyl functions. In some formulations this crosslinking species is a polymer of low molecular weight. The crosslinker copolymerizes with the monomers of the formulation to produce a network structure.
It is an object of the present invention provide macromolecular compounds which act as hydrophilic photocrosslinkers for a great number of different polymeric systems provided with functional groups for crosslinking or with vinylic, acrylic and methacrylic monomers in aqueous solution and thereby retaining the photoactive radical in the crosslinked network.
It is another object of the present invention to provide stable hydrophilic photocrosslinkers capable of being activated at wavelengths greater than 305 nm.
It is a further object of the present invention to provide photocrosslinkers with capability to act in aqueous solutions, especially on hydrophilic polymers or water-soluble macromolecular particles made thereof having functional groups for crosslinking.
It is a still further object of the present invention to provide photocrosslinkers with enhanced photoactivity (100% conversion of monomer to polymer in aqueous solution) which reduces photoinitiator residues to a minimum, especially, vinyl modifications of the photoinitiator component and thereby reducing compositional drift. Draize and other environmental hazards.
The invention as presented below will explain how the mentioned objects are met while discussing further obvious advantages.